1. Field of the Invention
The present invention relates to an automatic paper feeder for an image-scanning device, and more particularly to a pressure-adjustable mechanism for the paper feeding roller of the automatic paper feeder.
2. Description of the Prior Art
An automatic paper feeder is frequently included in various kinds of office automation equipment, such as image-scanning device, photocopier, printer, etc., so that paper sheets to be scanned, copied or printed could be automatically separated and fed via the automatic paper feeder. FIG. 1 illustrates a scanner 1 and a conventional automatic paper feeder 2 associated therewith. The scanner 1 is internally provided with an optical scanning module 11, a pair of guide bars 12, and related optical components, such as focusing lens, image sensor, driving mechanism, etc. The optical scanning module 11 is moved along the guide bars 12 under control of a control circuit of the scanner 1, and a sheet of paper 3 to be scanned is fed with the automatic paper feeder 2 for image scanning.
FIG. 2 shows an internal structure of the automatic paper feeder 2 of FIG. 1. The automatic paper feeder 2 mainly includes a paper tray 21, a paper feeding roller 22, a paper pressing plate 23, a stopping plate 24, a paper separating mechanism 25, and a paper separating plate 26. Paper sheets 3 to be fed are stacked on the paper tray 21 and held in place under a pressure applied by the paper pressing plate 23 on the paper sheets 3. Meanwhile, lower ends of the stacked paper sheets 3 are positioned against the stopping plate 24. A main function of the paper separating mechanism 25 is to provide a proper gap between the stopping plate 24 and the paper feeding roller 22 and to apply a proper pressure against the paper separating plate 26.
When the paper feeding roller 22 rotates, it brings a lowermost sheet of paper 3 in the paper tray 21 to move in a direction indicated by arrow I toward the gap between the paper feeding roller 22 and the paper separating plate 26. In the event two or more sheets of paper 3 are fed toward the gap, a difference between the friction coefficients of the paper-feeding roller 22 and the paper separating plate 26 would allow only the paper in contact with the paper feeding roller 22 to move through the gap. Therefore, the stacked paper sheets 3 could be separately fed for scanning one by one.
Conventionally, the paper tray 21 is a fixed-type structure. That is, the paper tray 21 is always fixedly mounted in the automatic paper feeder 2 independent of the quantity of paper sheet 3 on the paper tray 21. Meanwhile, the gap between the paper feeding roller 22 and the paper separating plate 26 is always constant, too. It is found, however, in actual operation of the conventional automatic paper feeder 2 having fixed paper tray 21, increased weight or amount of paper sheets 3 on the paper tray 21 would often have influences on accurate paper dispensing of the automatic paper feeder 2. Problems such as difficult paper feeding and paper dispensing occur when the weight or the amount of paper sheets 3 on the paper tray 21 increases.